Principals of Floatation ...
A balloon is like a submarine in water. When the Captain of the submarine, fills internal tanks with water, letting the air out, the water is heavier than air. The submarine becomes heavier than the surrounding water and sinks. The balloonist, fills his balloon with a gas lighter than air, such as Helium, and the balloon rises. He can then control how high he goes by letting out gas to go down or dropping some sand bags to go up. The balloon is acting like a submarine but it is in air instead of water.